The acoustic and perceptual effects of two noise-suppression algorithms

Abstract

Internal noise generated by hearing-aid circuits can be audible and objectionable to aid users, and may lead to the rejection of hearing aids. Two expansion algorithms were developed to suppress internal noise below a threshold level. The multiple-channel algorithm's expansion thresholds followed the 55-dB SPL long-term average speech spectrum, while the single-channel algorithm suppressed sounds below 45 dBA. With the recommended settings in static conditions, the single-channel algorithm provided lower noise levels, which were perceived as quieter by most normal-hearing participants. However, in dynamic conditions "pumping" noises were more noticeable with the single-channel algorithm. For impaired-hearing listeners fitted with the ADRO amplification strategy, both algorithms maintained speech understanding for words in sentences presented at 55 dB SPL in quiet (99.3% correct). Mean sentence reception thresholds in quiet were 39.4, 40.7, and 41.8 dB SPL without noise suppression, and with the single- and multiple-channel algorithms, respectively. The increase in the sentence reception threshold was statistically significant for the multiple-channel algorithm, but not the single-channel algorithm. Thus, both algorithms suppressed noise without affecting the intelligibility of speech presented at 55 dB SPL, with the single-channel algorithm providing marginally greater noise suppression in static conditions, and the multiple-channel algorithm avoiding pumping noises.

Aided cortical auditory evoked potentials in response to changes in hearing aid gain

“…Processing delay measurements using the Fonix 7000 (Frye Electronics; Tigard, OR) revealed a signal processing delay of 0.5 ms across the four gain settings. To differentiate sources of background noise measured in the ear canal, we completed aided coupler measurements comparing an unplugged microphone condition with a plugged condition to try and eliminate as much ambient noise as possible (Zakis & Wise 2006). Measurements for the two conditions were within 4 dB SPL at all gain settings, indicating that the majority of background noise added by the hearing aid was due to internal circuit noise rather than amplified ambient noise.…”

Effects of Expansion on Consonant Recognition and Consonant Audibility

“…For example, Plyler et al (2005a) measured in situ measurements of the effects of expansion on room noise and also on probe tube microphone measurements but not on the speech stimuli used in the study. Likewise, Zakis and Wise (2007) measured changes in audibility using the longterm speech spectrum as the input but did not measure changes in audibility with the specific speech tokens used in the study. Given that the long-term speech spectrum will be dominated by vowels, while recognition depends more heavily on consonant recognition, the relationship between expansion and consonant audibility needs to be better established.…”

“…Most of the Plyler et al (2005aPlyler et al ( , 2005b studies used a single-channel hearing aid with a kneepoint of 50 dB SPL and found decreased speech recognition. Three studies that used multichannel expansion still found reduced speech recognition (Walker et al, 1984;Plyler et al, 2007), even when the kneepoints were set at or below the LTASS for each frequency band (Plyler et al, 2007;Zakis and Wise, 2007). For this article, a 45 dB SPL LTASS will refer to kneepoints set to the LTASS in each channel for an overall speech input level of 45 dB SPL.…”

“…For this article, a 45 dB SPL LTASS will refer to kneepoints set to the LTASS in each channel for an overall speech input level of 45 dB SPL. Zakis and Wise (2007) set the kneepoints of a multichannel hearing aid to 10 dB below 45 dB SPL LTASS and 55 dB SPL LTASS. It was found that the high kneepoint level resulted in decreased speech recognition as measured by the Hearing in Noise Test, whereas the lower kneepoint did not.…”

Environmental noise reduction configuration: Effects on preferences, satisfaction, and speech understanding

“…The expansion ratio was set to 0.7:1 and the attack (rising level) and release (falling level) times were set to 20 and 200 ms, respectively, for all channels. This strategy of fitting multiple-channel expansion preserves the intelligibility of speech at the soft level of 55 dB SPL without inducing 'pumping' artefacts (Zakis & Wise, 2007;Wise & Zakis, 2008).…”